1. Field of the Invention
The present invention relates to a vehicle motion control device configured to secure stability of the vehicle by anti-skid brake system (ABS) control.
2. Description of Related Art
JP-A-Hei. 5-319238 discloses technology for removing instability caused by a difference in slip rate between left and right wheels of a vehicle during performing an ABS control on a split road surface where the left and right wheels are different from each other in friction coefficient (hereinafter, referred to as “road surface μ” or “μ”) of a traveling road surface of the vehicle. Since the split road surface has a difference in road surface μ between the left and right wheels, the left and right wheels have a difference in braking force generated. This braking force difference causes yaw moment to lead to instability of the vehicle. Therefore, in JP-A-Hei. 5-319238, a target slip rate of the wheel is set for the road surface where the road surface μ is higher (hereinafter, referred to as “high μ road”) such that the braking force difference is offset between the left and right wheels depending on a yaw rate deviation that is a difference between a target yaw rate and an actual yaw rate. Accordingly, the braking force on the side of the high μ road can be reduced, and thus the braking force of the wheels on the side of the high μ road approaches that of the wheel on the road surface where the road surface μ is lower (hereinafter, referred to as “low μ road”), so that it is possible to reduce the yaw moment caused by the braking force difference and thus to improve the vehicle instability.
In the ABS control, the pressure variation gradient of a wheel cylinder (hereinafter, referred to as “W/C”) in pulse pressure increasing, maintaining and reducing controls is set in advance regardless of the degree to which a driver steps a brake pedal. For this reason, timing when an actual slip rate exceeds the target slip rate set as mentioned above is based on vehicle speed variation changing depending on a preset pressure variation rate. Thus, although the yaw rate deviation occurs, the pressure reducing control is not started until the actual slip rate varying depending on a traveling environment exceeds the target slip rate. As a result, a difference in braking force between left and right wheels is generated, and thus high yaw moment is generated. Since a driver performs corrected steering (counter steering) to offset this yaw moment, opposite yaw moment is generated. Since this operation is repeated, it takes time to suppress an amplitude of the yaw rate deviation and an amplitude of a corrected steering angle in the corrected steering within a predetermined range. For this reason, it takes time until instability is removed, and thus a response characteristic is not sufficient.
Specifically, in the ABS control on the split road surface, the wheels on the side of the high μ on the split road surface are controlled while a slip rate control is generally performed by setting a slip rate, which is smaller than a slip rate at a μ peak in a relationship between the slip rate and the road surface μ, as the target slip rate. In a characteristic diagram (μ-s curve) showing the relationship between the slip rate and the road surface μ, the road surface μ is increased until the slip rate reaches a predetermined value. When the slip rate reaches the predetermined value, the road surface μ reaches a peak (μ peak). Afterwards, the road surface μ is reduced as the increase of the slip rate. For this reason, in the case where the slip rate, which is smaller than the slip rate corresponding to the μ peak, is set to the target slip rate, although the pressure reducing control is started after the slip rate reaches the target slip rate, the actual slip rate exceeds the target slip rate to reach a higher slip rate due to a time lag between the start determination and the actual start. Thus, the braking force difference between the left and right wheels is generated as described above, and thus high yaw moment is generated.
In the above, the case where the high yaw moment is generated on the split road surface has been described. However, the same problem occurs in turning of the vehicle. The situation where the vehicle is subjected to understeer (hereinafter, abbreviated to “US”) means the state where the vehicle exceeds a grip limit. This state is more likely to occur as the wheel slip becomes larger, and it is necessary to suppress the wheel slip to the target slip rate in the event of the US. Even in this case, the same problems as the case of the above-mentioned split road surface occur. For example, the pressure reducing control is not started until the actual slip rate exceeds the target slip rate.